The present invenion relates to a new and improved construction of a tread for a pneumatic tire which is of the type comprising two inner ribs extending in the circumferential direction of the tire and, to both sides of said ribs, tiers or block rows likewise extending in the circumferential direction of the tire, the block rows and ribs being separated from one another in each case by a respective zig-zag circumferential groove.
Such type tires are designed for use during winter driving conditions and over relatively rough terrain or land. By providing a rather coarse pattern of the tire profile such tires have improved traction in snow. The snow which is compressed by the pressure of the tire thus forms, within the profile grooves, a particularly highly compacted structure or rib configuration, so that, similar to the transmission of forces between a gear and rack, the engine power of the vehicle is transmitted to the road surface or the like extensively free of loss.
Heretofore known constructions of the previously mentioned type of tire treads possess three identical zig-zag configured circumferential grooves. Since at the region of the tread or contact surface of the tire the tread pressure of passenger vehicle tires acting upon the roadway surface is greater at the shoulder region than at the center of the tread, there arises a different degree of compaction of the snow in the aforementioned circumferential grooves. As a consequence thereof, the traction is reduced, since, as a practical matter, only both of the outer or lateral circumferential grooves are effective.